Hi all,
Just thought I'd share my experience! Purchased an IS250 last month, was 3 weeks out of warranty when I purchased it and purchased outside of the Lexus network. Car had a full service history every 10K miles from Lexus Milton Keynes and had alloys replaced under Warranty 18 months ago. Noticed these ones were also showing similar signs of blistering.
Now registered the car at Lexus Bracknell who logged a call to head office and they said they wouldn't be able to repalce them as a goodwill gesture because the car wasn't bought from Lexus. Bit annoying as I would have thought a set of alloys that were replaced 18 months ago would be subject to goodwill as that is by far the quickest corroison rate I've heard of. They said the alloys would have only had warranty for a year and there is no defect with them as it's purely a cosmetic issue?!
Quite annoying as when I bought the car it had only just been serviced by Lexus, just happened to be at a non Lexus Dealer! Initially they said aslong as the car had been serviced with Lexus every 10K miles then it would be fine - however head office then came back and said no because I purchased the car outside of Lexus.
Best of luck getting them replaced though - you should be ok if your in warranty!